1. Field of the Invention
This invention pertains to thermal barriers, and more particularly to transparent thermal barriers for protecting personnel from fire.
2. Description of the Prior Art
Aircraft canopies or the like commonly are made from transparent organic material such as polymethyl methacrylate. This material is more commonly known as plexiglass, a light weight thermoplastic polymer of methyl methacrylate. Aircraft canopies must be sufficiently rigid to withstand wind pressure, and consequently have significant thickness when manufactured of plexiglass. However, plexiglass has a relatively low rate of heat transfer and a relatively large coefficient of thermal expansion, as is characteristic of organic materials. Thus when an aircraft having a plexiglass canopy is exposed to a fire, such as may occur on the deck of an aircraft carrier, the flames heat the outer surface of the canopy and cause it to expand while the inner surface stays cool, thereby inducing thermal stresses in the plexiglass canopy. This results in fracture of the canopy, and exposure of the pilot to the flames.
A canopy made of an inorganic material such as glass, would have improved thermal characteristics but would exact a severe weight penalty on the aircraft. Also, an all glass canopy would present a severe splinter hazard to the pilot if it were damaged by enemy gun fire, or other mechanical or thermal means.